The present invention will be described with particular reference to fractionation of a lignocellulosic material to obtain cellulose in a form suitable for enzymatic hydrolysis and fermentation to ethanol. However, it will be appreciated that the fractionated products obtained by the methods of the present invention may have other end uses and no limitation is intended thereby.
Cellulose, together with lignin and hemicellulose, is found in the cell wall of plants. Plant cell wall material is also known as biomass or lignocellulose. The terms biomass and lignocellulose have the same meaning here. Cellulose is primarily used to manufacture pulp and paper products, textiles and fibres. Cellulose has been proposed as an alternative source of its component sugar glucose. Glucose may be readily fermented to ethanol which is considered a valuable biofuel. Ethanol is currently produced from hydration of ethylene from petroleum stocks or from natural starches from corn and sugar from sugar cane or beet. Starch and sugar are relatively easily hydrolysed to their component monosaccharides. However, corn and sugar are also a valuable food source and product on their own right. It would be desirable to be able to obtain ethanol from a non-food source and in particular a waste product. The use of cellulose as a source of “bio-ethanol” has therefore been proposed.
Hydrolysis of cellulose to glucose may be carried out chemically or enzymaticly. Chemical hydrolysis involves the use of concentrated acids such as sulphuric acid. Acid hydrolysis is economically unfavourable in view of the costs of the acids. Enzymatic hydrolysis offers a more favourable economic result, but only if the enzymes are able to be continuously recycled or used at suitably low dose rates. However the presence of lignin and hemicellulose to some extent deactivate cellulase enzymes. Thus, separation of these components from cellulose is important for an efficient and economically viable process. Furthermore, cellulose in its native state in fibrous plants is partially crystalline and this crystalline component resists enzymatic hydrolysis. Current commercial methods for fractionating lignocellulosic material such as the Kraft process either do not sufficiently separate the lignin from cellulose or produce cellulose with most of the crystalline component preserved. Cellulose sourced from these processes is thus not well suited for enzymatic degradation. A further disadvantage of these commercial processes is that they generate undesirable pollutants. Consequently there is considerable interest in obtaining an environmentally acceptable and commercially viable method of fractionating lignocellulosic materials.
Cellulose is known to be able to be dissolved in ionic liquids. Ionic liquids are organic salts which exist as liquids at relatively low temperatures. Currently, cellulose processing and chemistry relies primarily on carbon disulfide and caustic bases as dissolving solutions. The efficiency of existing methods for dissolving and derivatizing cellulose can be significantly improved by the availability of suitable solvents for refined and natural cellulose. Solutions of cellulose and ionic liquids are amenable to conventional processing techniques for the formation of cellulose threads, thin films, and beads. For example, dyes, as well as complexants for coordination and binding of metal ions, that are insoluble in water can be readily dissolved in this polar ionic liquid at high concentration. In this way, they can be integrated into a processed hydrophilic cellulose matrix to obtain materials suitable for sensing and remediation in aqueous media.
Cellulose dissolved in an ionic liquid is recovered according to the conventional polymer chemistry technique of adding a miscible non-solvent to a polymer solution. The desired cellulose polymer forms a precipitate in the single phase and can be recovered by conventional liquid/solid separation techniques. Polymers precipitated in this manner are generally contaminated with the dissolving liquid which must be separated from the precipitated material. Such separation is known to be difficult in the case of cellulose precipitated from an ionic liquid. Further, ionic liquids are quiet expensive which means that in order for a process to be commercially viable, the ionic liquid must be able to be recycled in a relatively efficient manner. However, separation of the miscible non-solvent has also proven difficult. The present inventors are unaware of any commercial process for processing cellulose that involves the use of ionic liquids.
The present invention relates to a method of fractionating a lignocellulosic material and obtaining cellulose therefrom.